The present invention relates to drag-reducing polymers and their method of manufacture. More specifically, this invention relates to a method for preparing ultra-high molecular weight, substantially non-crystalline hydrocarbon-soluble polymers through bulk polymerization.
A drag-reducing agent is one that substantially reduces the friction loss that results from the turbulent flow of a fluid. Where fluids are transported over long distances, such as in oil and other hydrocarbon liquid pipelines, these friction losses result in inefficiencies that increase equipment and operations costs. Ultra-high molecular weight polymers are known to function well as drag-reducing agents, particularly in hydrocarbon liquids. In general, drag reduction depends in part upon the molecular weight of the polymer additive and its ability to dissolve in the hydrocarbon under turbulent flow. Effective drag-reducing polymers typically have molecular weights in excess of five million.
Drag-reducing polymers are known in the art. Representative, but non-exhaustive, samples of such art are: U.S. Pat. No. 3,692,675, which teaches a method for reducing friction loss or drag for pumpable fluids through pipelines by adding a minor amount of a high molecular weight, non-crystalline polymer; and U.S. Pat. No. 3,884,252, which teaches the use of polymer crumb as a drag-reducing material. These materials are extremely viscoelastic, and, in general, have no known use other than as drag-reducing materials. However, the very properties that make these materials effective as drag-reducing additives make them difficult to handle because they have a severe tendency to cold flow and reagglomerate even at subambient temperatures. Under conditions of pressure, such as stacking or palleting, cold flow is even more intense and reagglomeration occurs very quickly.
The general propensity of non-crosslinked elastomeric polymers (elastomers) to cold flow and agglomerate is well-known. Polymers of this sort cannot be pelletized or put into discrete form and then stored for any reasonable period of time without the materials flowing together to form large agglomerates. Because of such difficulties, elastomers are normally shipped and used as bales of rubber. However, such bales must be handled on expensive equipment and cannot be pre-blended. In addition, polymers such as the drag-reducing additives described are not susceptible to such balings, since cold flow is extremely severe. Further, dissolution time for such drag-reducing materials from the bulk polymer state in the flowing hydrocarbons to a dissolved state is so lengthy as to severely reduce the effectiveness of this material as a drag-reducing substance.
Numerous attempts have been made to overcome the disadvantages inherent in cold-flowing polymers. Representative, but non-exhaustive, of such art is that described in U.S. Pat. No. 3,791,913, wherein elastomeric pellets are surface cured, i.e., vulcanized to a minor depth in order to maintain the unvulcanized interior of the polymer in a xe2x80x9csackxe2x80x9d of cured material, and U.S. Pat. No. 4,147,677, describing a method of preparing a free-flowing, finely divided powder of neutralized sulfonated elastomer by admixing with fillers and oils. This reference does not teach a method for making free-flowing powders of non-elastomeric material. U.S. Pat. No. 3,736,288 teaches solutions of drag-reducing polymers in inert, normally liquid vehicles for addition to liquids flowing in conduits. A xe2x80x9cstaggered dissolutionxe2x80x9d effect is provided by varying the size of the polymer particles. Suspension or surface-active agents can also be used. While directed to ethylene oxide polymers, the method is useful for hydrocarbon-soluble polymers as well. U.S. Pat. No. 4,088,622 describes a method of making an improved, molded drag-reducing coating by incorporating antioxidants, lubricants, and plasticizers and wetting agents in the form of a coating which is bonded directly onto the surface of materials passing through a liquid medium. U.S. Pat. No. 4,340,076 teaches a process for dissolving ultra-high molecular weight hydrocarbon polymer and liquid hydrocarbons by chilling to cryogenic temperatures comminuting the polymer formed into discrete particles and contacting these materials at near cryogenic temperatures with the liquid hydrocarbons to more rapidly dissolve the polymer. U.S. Pat. No. 4,341,078 immobilizes toxic liquids within a container by injecting a slurry of cryogenically ground polymer particles while still at cryogenic temperatures into the toxic liquid. U.S. Pat. No. 4,420,440 teaches a method for collecting spilled hydrocarbons by dissolving sufficient polymer to form a nonflowing material of semisolid consistency by contacting said hydrocarbons with a slurry of cryogenically comminuted ground polymer particles while still at cryogenic temperatures.
Some current drag-reduction systems inject a drag-reducing polymer solution containing a high percentage of dissolved, ultra-high molecular weight polymer into conduits containing the hydrocarbon. The drag-reducing polymer solution is normally extremely thick and difficult to handle at low temperatures. Depending upon the temperature of the hydrocarbon and the concentration at which the drag-reducing polymer solution is injected, significant time elapses before dissolution and resulting drag reduction. Solid polymers of these types can take days to dissolve in some cases, even though drag reduction is greatly enhanced once dissolution has finally occurred. Dissolution at low temperatures, i.e., below 70 F, may be so slow with some polymers that there is little practical drag-reduction benefit with those polymers. Further, certain polymers may not be suitable for certain hydrocarbons. For instance, copolymers of xcex1-olefins will not dissolve in heavy, aromatic crude oils and so are unsuitable in these applications as drag-reducing agents.
Another way to introduce ultra-high molecular weight polymers into the flowing hydrocarbon stream is through a suspension. Powdered ultra-high molecular weight polymers are suspended in a liquid that will not dissolve or will only partially dissolve the ultra-high molecular weight polymer. This suspension is then introduced into the flowing hydrocarbon stream. The tendency of the ultra-high molecular weight polymers to reagglomerate makes manufacture of these suspensions difficult. A way of controlling the tendency of the ultra-high molecular weight polymers to reagglomerate is to partially surround the polymer particles with a partitioning agent, occasionally termed a coating material, to reduce the ability of these polymers to reagglomerate. U.S. Pat. No. 4,584,244, which is hereby incorporated by reference, describes a process whereby the polymer is ground and then coated with alumina to form a free-flowing powder. Some processes using a partitioning agent require that the partitioning agent completely surround the polymer core, which requires that at least 20% and often as much as 50% of the weight of the final composition be the partitioning agent. Other examples of partitioning agents used in the art include talc, tri-calcium phosphate, calcined clays, magnesium stearate, silica, polyanhydride polymers, sterically hindered alkyl phenol antioxidants, and graphite. Partitioning agents, however, add weight to the drag-reducing agent material, resulting in higher transport costs and additional handling equipment, without any drag-reducing benefit. Further, some partitioning agents are incompatible with the hydrocarbon fluid or may be an unwanted contaminant in the hydrocarbon fluid.
The most common polymerization method for manufacturing these polymers is solution polymerization. With solution polymerization, however, yields are often lower than desired and the solution medium must be removed, often using an expensive and time-consuming drying process, before the polymer may be used. In addition, when using solution polymerization processes to produce drag-reducing polymers it is necessary to terminate reactions at no more than 20% polymer based on total reactor content by weight in order to obtain the high molecular weight polymers in an effective form, as taught in U.S. Pat. Nos. 4,415,714, 4,493,903, and 4,945,142.
One alternative to solution polymerization is bulk polymerization. Bulk-polymerized drag-reducing polymers should have five characteristics: 1) less than 5% unpolymerized material should remain in the bulk polymerized material; 2) it must be possible to grind them to a fine, stable powder; 3) they must form stable suspensions; 4) they must dissolve rapidly in various hydrocarbons at temperatures as low as 30xc2x0 F.; and 5) they must effectively reduce the frictional drag in hydrocarbon pipelines.
Accordingly, a drag-reducing polymer, and drag-reducing polymer suspension and method of manufacturing the drag-reducing polymer suspension are disclosed herein that meet all five of these criteria. One embodiment of the present invention is drawn to an ultra-high molecular weight copolymer composed of a vinyl aromatic monomer and an xcex1-olefin monomer where the xcex1-olefin monomer has a carbon chain length of between 2 and 20 carbon atoms. In another embodiment, a method of manufacturing an ultra-high molecular weight copolymer is disclosed where a mixture of an xcex1-olefin having a carbon chain length of between 2 and 20 carbon atoms and a vinyl aromatic is bulk polymerized to form the ultra-high molecular weight copolymer. The ultra-high molecular weight copolymer has a molecular weight of greater than 1 million. In still another embodiment, method of manufacturing an ultra-high molecular weight copolymer is disclosed where the ultra-high molecular weight copolymer is manufactured by forming a monomer mixture composed of an xcex1-olefin monomer having a carbon chain length of between 2 and 20 carbon atoms and a vinyl aromatic monomer. An initiator, catalyst and promoter are then added and the mixture allowed to react to form the ultra-high molecular weight copolymer. The ultra-high molecular weight copolymer has a molecular weight of greater than 1 million. In another embodiment of the present invention, a method for the preparation of a drag-reducing copolymer suspension is disclosed where an ultra-high molecular weight copolymer is prepared by co-polymerization of an xcex1-olefin with a vinyl aromatic, wherein the xcex1-olefin has a carbon chain length of between 2 and 20 carbon atoms. The ultra-high molecular weight copolymer is then ground at a temperature below the glass-transition temperature of the ultra-high molecular weight copolymer to form ground polymer particles. The ground polymer particles are then mixed with a suspending fluid to form the drag-reducing polymer suspension. In still another embodiment, a drag-reducing polymer solution is disclosed, which is composed of an ultra-high molecular weight copolymer having a vinylaromatic monomer and an xcex1-olefin monomer having a carbon chain lengths of between 2 and 20 carbon atoms, and a heavy hydrocarbon.
One advantage of the present invention is that the drag-reducing polymer suspension is easily transportable and does not require pressurized or special equipment for storage, transport, or injection. Another advantage is that the drag-reducing polymer is quickly dissolved in various flowing hydrocarbon streams. Yet another advantage is that the copolymers of the present invention are soluble in heavy aromatic hydrocarbons at temperatures as low as 0xc2x0 C. (32xc2x0 F.). Another advantage of copolymers of the present invention is that they may either be transported in a stable suspension, or pre-dissolved in a small amount of heavy hydrocarbon. Still another advantage of the present invention is that reagglomeration of the drag-reducing polymers is greatly reduced, allowing for easier handling during manufacture. Another advantage of the present invention is that the drag-reducing polymer suspension is stable, allowing a longer shelf life and balancing of customer demand with manufacturing time. Another advantage is that costly solvent removal is minimized. A further advantage of the present invention is that the amount of inert ingredients in the final product is reduced.